1. Technical Field
The present invention relates to a shadow mask for a cathode ray tube and, in particular, to a shadow mask for a cathode ray tube which enhances display image quality by defining the length of beam passage holes formed at the periphery of the effective screen area within a suitable range, thereby reducing the non-light emitting region while making the light emitting uniform.
2. Related Art
Generally, a cathode ray tube includes an electron gun for emitting electron beams, a deflection yoke for deflecting the electron beams, a shadow mask for color-selecting the electron beams, and a panel with an inner phosphor film. The electron beams emitted from the electron gun are deflected by the deflection magnetic fields from the deflection yoke, and the deflected electron beams pass through the color-selecting shadow mask and then collide against green, blue and red phosphors of the phosphor film, thereby emitting light to display the desired images.
The shadow mask color-selectively passes the electron beams, and correctly lands them on the relevant phosphors of the phosphor film. For this purpose, beam passage holes with a predetermined pattern are formed in the shadow mask to pass the electron beams.
The shadow mask is commonly demarcated into an effective screen portion having the beam passage holes and an inactive portion having no holes. The beam passage holes are arranged in a zigzag manner so that the bridges at the neighboring columns are not placed on the same line.
Accordingly, when beam passage holes with a full length are formed at any one column from the division between the effective screen portion and the inactive portion, the length of the beam passage holes placed at the column adjacent to the former column close to the division is set to ½ of the full length.
Although the phosphor film formed on the panel is wider in area than the effective screen portion, the electron beams passed through the beam passage holes at the division do not reach the edge of the phosphor film so that the non-light emitting area of the phosphor film is increased, and the display image quality is deteriorated. That is, the non-light emitting area where the phosphor film does not emit light exists at the edge portion of the phosphor film and has a large width.